1. Field of the Invention
The invention relates to a petroleum refining process. More particularly, the invention relates to a coking process for converting petroleum based feedstocks to coke, hydrocarbon liquids and gases. Most particularly the invention relates to converting a sulfur containing residual hydrocarbon feedstock to low sulfur coke.
2. Description of other Related Methods in the Field
In a coking process, a heavy liquid hydrocarbon fraction is converted to solid coke and lower boiling liquid and gaseous products. The heavy liquid hydrocarbon fraction is typically a residual petroleum based oil or a mixture of residual oil with other heavy hydrocarbon fractions.
In a typical delayed coking process, the residual oil is heated by exchanging heat with liquid products from the process and is fed into a fractionating tower wherein light end products are removed from the residual oil. The residual oil is then pumped from the bottom of the fractionating tower through a tube furnace where it is heated under pressure to coking temperature and discharged into a coking drum.
In the coking reaction residual oil feedstock is thermally decomposed into solid coke, condensable liquid and gaseous hydrocarbons. The liquid and gaseous hydrocarbons are continuously removed from the coke drum and returned to the fractionating tower where they are separated into the desired hydrocarbon fractions.
When the coke drum becomes filled with coke, the flow of feedstock is terminated and solid coke is recovered from the coking drum. Coke quality determines its use. Two grades of high purity coke are used to manufacture electrodes for the steel and aluminum industry. Lower purity coke is used for fuel. The value of lower purity coke is calculated based on the sulfur and heavy metal impurities which are transferred from the feedstock to the coke.
Premium coke is a high purity grade of coke used for the manufacture of large graphite electrodes used in electric arc furnaces for the production of steel. The quality of premium coke is measured by its coefficient of thermal expansion (CTE) which may vary from as low as 0 to as high as 8.times.10.sup.-7 centimeters per centimeter per degree centigrade. The best premium grade coke has a CTE of 5.times.10.sup.-7 cm/cm/.degree.C. or less.
Aluminum grade coke is another high purity grade of coke used for the manufacture of electrodes for the production of aluminum. Aluminum grade coke is of lesser purity than premium grade coke and contains amounts of sulfur and nitrogen. The CTE of aluminum grade coke is also substantially higher than the requirement of premium grade coke.
U.S. Pat. No. 5,045,177 to J. C. Cooper et al. teaches a delayed coking process in a residual oil feedstock is converted to coke, liquid hydrocarbon and a sweet gas fraction.